The invention relates to a perforating gun.
For purposes of causing well fluid to flow from a producing formation into a well, a perforating gun may be lowered downhole into the well and detonated to pierce a casing (of the well) and form fractures in the formation. After the perforating gun detonates, well fluid typically flows into the casing and to the surface of the well via a production tubing that is located inside the casing. A seal typically is formed (by a packer, for example) between the inside of the casing and the exterior of the production tubing, and the well fluid enters the production tubing from beneath this seal.
The production tubing typically is set in place before the perforating gun is lowered downhole. As a result, the perforating gun must be lowered down through the central passageway of the production tubing to access a lower section of the well casing (beneath the production tubing) for purposes of piercing the casing and forming the fractures. Therefore, at least when passing through the production tubing, the maximum cross-sectional diameter of the perforating gun is limited by the inner diameter of the production tubing.
The size restriction imposed by the production tubing may limit the size of shaped charges (i.e., the high explosives) of the perforating gun unless the gun has a mechanism to cause the longitudinal axes of the shaped charges to become aligned with the longitudinal axis of the production tubing when the charges pass through the tubing. After passing through the production tubing, the mechanism may radially expand, or deploy, the charges. Therefore, if the gun does not include this alignment mechanism, the size restrictions imposed by the inner diameter of the production tubing may limit the size and thus, the amount of explosives that are placed downhole.
Besides maximizing the amount of explosives that are lowered downhole, the performance of the perforating gun may be enhanced in other ways. As an example, performance of the perforating gun may be enhanced by minimizing a radial standoff distance between the charges and the portion of the casing where perforation occurs. However, the radial deployment of the charges (after passing through the production tubing) typically reduces the standoff distances. As another example, performance of the perforating gun may be enhanced by increasing the shot density (i.e., decreasing the distance between adjacent charges) of the perforating gun.
As an example of the many different types of perforating guns, in one type of perforating gun (often called an "Enerjet gun"), charges are secured to a loading strip. For example, the charges may be secured to recesses of the loading strip by support rings. The cross-sectional diameter of the Enerjet gun is equal to or smaller than the inner diameter of a production tubing. However, the charges of the Enerjet gun are not radially deployed after passing through the production tubing, but rather, the charges are permanently fixed in radially outward directions. As a result, the longitudinal dimension of each charge, the standoff distances and the amount of explosives of the gun are limited by the inner diameter of the production tubing. Furthermore, the Enerjet gun does not include a mechanism to increase the shot density of the gun once the gun passes through the production tubing. In a second type of perforating gun (often called a "Hyperdome gun") similar in some aspects to the Enerjet gun, shaped charges arc packaged in a hollow carrier tubing that has an outer diameter which is smaller than the inner diameter of the production tubing. However, the Hyperdome gun typically has the same limitations as the Enerjet gun.
In a third type of gun (often called a "Pivot gun"), charges are connected to a carrier tubing and are radially deployed after being run through the production tubing. While being run through the production tubing, the longitudinal axes of the charges are aligned with a longitudinal axis of the production tubing, and as a result, for purposes of running the gun downhole, the cross-sectional diameter of the Pivot gun is smaller than or equal to the inner diameter of the production tubing. During deployment of the charges, sets of linkages rotate the charges in radially outward directions to their shooting positions. Therefore, the Pivot gun has a mechanism to deploy and orient charges to fulfill the purposes of increasing charge sizes and decreasing standoff distances. However, the Pivot gun does not include a mechanism to increase the shot density of the gun after deployment of the charges. In another type of perforating gun (often called a "Swingjet gun"), charges are connected to a carrier tube and deployed in a similar manner to the Pivot gun. Similar to the Pivot gun, the Swingjet gun does not have a mechanism to increase the shot density of the gun after the charges are deployed.
In a fifth type of perforating gun, charges arc connected to each other at their two ends, instead of being connected to a carrier tube. A connecting bar is filled with an explosive that transfers a detonation from charge to charge. Two cables are used to set the position of the bottom charge. Once this is done, the positions of the rest of the charges are set by gravity. However, because of this type of gravity-induced mechanism, the perforating gun may only be used in vertical or near-vertical wells.
Thus, there is a continuing need for a perforating gun that minimizes the distances between deployed charges regardless of the spatial orientation of the gun.